An inertial frame of reference is one which satisfies newtons first law. Suppose I throw a ball it moves because there is a force acting on the ball. Now non inertial reference frame would be one in which you are in a bus and the bus moves forward or accelerates due to this you are pushed backward towards your seat.

Now you cannot say that there is no force acting on you. This force is the result of the bus moving forward and should obey newtons third law every action has an equal and opposite reaction. Action- bus moving forward, Reaction- You moving towards your seat.

An inertial frame of reference is one which satisfies newtons first law. Suppose I throw a ball it moves because there is a force acting on the ball. Now non inertial reference frame would be one in which you are in a bus and the bus moves forward or accelerates due to this you are pushed backward towards your seat.

Now you cannot say that there is no force acting on you. This force is the result of the bus moving forward and should obey newtons third law every action has an equal and opposite reaction. Action- bus moving forward, Reaction- You moving towards your seat.

So how can this be a non inertial reference frame?

I think, like many people, you might have the wrong idea of what an inertial frame of reference is. An inertial frame of reference is a frame of reference in which a body at rest stays at rest and a body in motion travels in a strainght line at constant speed. If you change to a frame of reference which is accelerating with respect to an inertial frame then the acceleration of the objects you observe are due to what are known as inertial forces which is a force which causes an acceleration which is independent of the mass of the body. The gravitational force is an example of an inertial force. In fract this is the very idea that led Einstein to his general theory of relativity. You can read more about it on my webpage at:

"PMB" if you remember we had a discussion about inertial frame of reference and that thread cleared most of my doubts. I know you have to answer many questions so you might not remember but can you tell me where I have misunderstood inertial frame of reference?

An inertial frame of reference is one which satisfies newtons first law. An object at rest will continue to be at rest and an object in motion will continue to be in motion unless acted upon by an external force. See the part which says unless acted by an external force. If I throw a ball the ball moves due to an external force satisfying newtons first law so this is an inertial frame of reference even though the ball is accelerating.

Now I was thinking that when a bus accelerated and you are in it you are pressed towards your seat I thought newtons third law is in action which provides the force but newtons third law requires two bodies to interact in order to produce an effect and the force which makes you press towards your seat is a fictitious force and there is no interaction between two bodies. Fictitious forces are also called inertial forces.

As far as gravity being a force or not, in relativity gravity is not a force but in classical mechanics it is a force.

I dont know whether I should ask another question but i have a doubt as to why fictitious forces are called inertial forces if you literally see the definition of inertia it says resistance to motion and a commoner would understand inertial forces to mean the force which resists motion so why the name inertial forces and as you said inertial forces dont depend on mass of the object but inertia depends on mass.

An inertial frame of reference is one which satisfies newtons first law. Suppose I throw a ball it moves because there is a force acting on the ball. Now non inertial reference frame would be one in which you are in a bus and the bus moves forward or accelerates due to this you are pushed backward towards your seat.

Now you cannot say that there is no force acting on you. This force is the result of the bus moving forward and should obey newtons third law every action has an equal and opposite reaction. Action- bus moving forward, Reaction- You moving towards your seat.

So how can this be a non inertial reference frame?

I don't understand your question. It seems obvious to me that a ball at rest on the floor of the bus will, when the bus accelerates, roll toward the back of the bus. Therefore, by what you are saying, this is NOT an inertial frame.

In my first example the ball is not thrown on the floor of the bus but it is thrown on the floor of the ground in an open space. The bus example is in the second case when you are commuting, Got it HOI?

Yes, I "got it" and I understood that initially. MY previous post still stands. I do not understand why you define "inertial frame", give an example that clearly does NOT fit that definition and them ask "why is that not an inertia frame?" Hadn't you pretty much just answered that question?

As PMB mentioned that inertial forces are not dependent on mass so you could reason out that F=ma. So Force is proportional to mass if gravity is a force as you mentioned it is not. So by this logic gravity is not a force.

But if you see in classical mechanics gravity F = Gm1m2/r2, force depends on mass, it is proportional to mass of the 2 objects and inversely proportional to radius. So gravity is a force.

In classical mechanics:
Force is a vector, which means it has magnitude and direction.
The above formula refers only to the magnitude.
The complete version also specifies the direction

The force, F, = Gm1m2/r2 and acts along the direct line connecting the centres of m1 and m2. r is also measured along this line

So any in motion involving gravity there are two cases.

1) The motion is parallel to the line connecting m1 and m2 and therefore parallel to r which therefore varies. Thus F must vary in magnitude according to the formula.

2) The motion is not parallel to the line connecting m1 and m2 and therefore the direction of the line between m1 and m1 must be constantly varying as the body moves, in accordance with the direction condition.

Either way the force F must change, which constitutes an acceleration.
Of course in case(2) the magnitude of F may also change or it may not, depending on the path of the motion.

This made my understanding better. This helped me. I like this forum because this is not a question answer forum where if i have a doubt i have to make another thread i can continue the discussion and one more advantage is that many forums have rules which state if you google and find the answer then dont ask. But as Hallsofivy observed I answered my own question so i could have not put it up here but look when i put it up here you told me the formula was incomplete so if i had kept the knowledge to myself i wouldnt have known this so it is important to discuss.